I don't know what to tell you about this kind of situation. If someone is intentionally feeding the other team might be killing him just to keep him out of the way. And while you can say in All-chat that X player is feeding, they have no way to prove that you are telling the truth. For all they know it could be a ruse. (a very dumb one though.)
I wish I could suggest something, but my mind is drawing a blank. I would never intentionally feed and can't really imagine why someone would, so to me the problem is I can't come up with a way to adequately punish feeders, which really is what is necessary here. There needs to be a way to permanently stop this behavior.
The only idea I come up with is if 4 players report the 5th for reporting the system should check their KDA rating. If its below a certain threshold the account gets an immediate 2 week ban. If it happens 3 times the account is deleted. No human involvement. Make it so 1 time feeding blocks the account for a week and 3 is it.
Yes players might have a bad day, but just set a known limit so that player who are struggling can either park at the fountain and stop hurting their team or have a minimum number of kills/assists to prevent the account suspension so they can hopefully get a bit of help in the match and improve their score above the threshold. Honestly, even in my worst games I usually pick up 3 assists just by being involved in enough team fights, an intentional feeder would find his team unwilling to get into team fights when he is near just to keep his score below the threshold. yes they probably still lose, but feeders would become very rare extremely quickly.